The effect of light intensity on hydrogen production by sulfur-deprived Chlamydomonas reinhardtii

The effect of light intensity on hydrogen production by sulfur-deprived Chlamydomonas reinhardtii was studied in situ using either long- or short-term experiments, or alternatively, with samples withdrawn from the photobioreactor. Overall hydrogen production by S-deprived culture was shown to depend on the light intensity and to exhibit regions of light limitation and light inhibition. The optimal incident light intensity for hydrogen production was independent of the method of sulfur deprivation or the initial acetate concentration in the medium (12-34 mM). However, it varied with the Chl concentration and the thickness of the photobioreactor. To calculate the average light intensity in the photobioreactor under different experimental conditions, a special mathematics approach was developed. The optimal average light intensity for H(2) production appeared to be 30-40 microE m(-2)s(-1) and was independent of the Chl or acetate concentrations and the method of S deprivation. The inhibitory effect of high light intensity was related to the enhanced O(2) evolution activity during the photosynthetic stage of sulfur deprivation and to the high activity of photosystem II at the beginning of the H(2)-production phase. Data support the major role of photosystem II in supplying reductants through photosystem I to the hydrogenase throughout the H(2)-production phase.